The present invention relates generally to magnetic resonance imaging (MRI), and more particularly to, a method and apparatus to rapidly acquire T2 weighted MR images with a steady state pulse sequence.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or "longitudinal magnetization", M.sub.z, may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.t. A signal is emitted by the excited spins after the excitation signal B.sub.1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (G.sub.x G.sub.y and G.sub.z) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
One of the problems with acquiring T2 weighted images is that conventional techniques require a relatively long repetition time (TR) in order to achieve a pure T2 (proton density) image contrast. In other words, T2 weighted imaging is generally associated with long imaging times, as compared to T1 weighted imaging. T1 imaging is much faster since such imaging is achieved by shortening the repetition time.
Rapid imaging methods and sequences typically use short pulse repetition times and gradient reversal echoes. Often pulse repetition times are comparable with the transverse relaxation time T2, whereby the transverse magnetization is not destroyed between phase encoding cycles. The common factor between most of the prior art rapid imaging sequences is the application of a closely spaced train of excitation pulses with a flip angle, usually equal to or less than 90.degree.. The signal is sampled between the pulses, following phase encoding dephasing/rephasing by a frequency encoding or view gradient. However, changes in the various gradient patterns will lead to significant differences in the resulting steady state signal, and image contrast. The different variants of the basic rapid acquisition concept may be categorized as either spoiling or refocusing sequences, depending on whether the transverse magnetization component contributes to the steady state magnetization. In spoiling, the transverse magnetization is destroyed between cycles, and only the longitudinal magnetization component contributes to this steady state.
To preserve the steady state, which includes the transverse magnetization, the effect of incrementing the phase encoding gradient must therefore be canceled. This can be done by applying a gradient of opposite area in each cycle, between data collection and the onset of the next pulse, which has led to the development of the refocusing sequences. In prior art spoiling sequences, the signal intensities are strongly T1 weighted, and it is not possible to obtain such pure T1 weighted contrast by using other rapid sequences. The refocusing sequences create contrast which are neither pure T1 nor pure T2 weighted, but are more of a ratio of T1 and T2 weighting. For similar reasons, it is difficult to obtain pure T2 weighted contrast by the refocusing technique.
Other prior art rapid imaging schemes using steady state coherent imaging methods can acquire two images simultaneously. However, one image is a mixed T1 and T2 contrast image, and the other is similar to the first with an additional T2 weighting. With appropriate sequence parameters, the ratio of the two images can approach a pure T2 weighted image, but they are not pure T2 weighted images, and therefore are less desirable. For good T2 weighting, the flip angle must be high, preferably near 90.degree.. However, since there is a distribution of flip angles throughout a slice, the contrast is quite variable throughout the slice with such methods. With 3-D imaging, this is theoretically less of a problem in central slices where the flip angles can be closer to 90.degree.. However, even with 3-D imaging, the resulting images have been less than desirable.
It would therefore be desirable to have a method and apparatus capable of rapid T2 weighting of MR images in times that approximate T1 imaging that can function in steady state and provide pure T2 weighted contrast images, and obtain pure T1 contrast.